The casino magnate Sheldon G. Adelson told Donald J. Trump in a private meeting last week that he was willing to contribute more to help elect him than he has to any previous campaign, a sum that could exceed $100 million, according to two Republicans with direct knowledge of Mr. Adelson’s commitment.

As significant, Mr. Adelson, a billionaire based in Las Vegas, has decided that he will significantly scale back his giving to congressional Republicans and direct most of his contributions to groups dedicated to Mr. Trump’s campaign. The two Republicans familiar with Mr. Adelson’s plans spoke anonymously because they were not authorized to discuss the matter publicly.

Mr. Adelson’s pledge to Mr. Trump, the presumptive Republican presidential nominee, comes at an opportune time. Mr. Trump has relied on a mix of his own wealth and small-dollar contributions to finance his primary effort and lacks the sort of major donor network needed to sustain him in the general election. Mr. Trump has said that he may need $1 billion for the campaign but has only recently begun scheduling fund-raisers and hiring finance staff members. Many of the Republican Party’s wealthiest contributors, including the billionaire brothers Charles G. and David H. Koch, have indicated they are unlikely to give to his candidacy.

What remains unclear is how Mr. Adelson plans to contribute his money to Mr. Trump. He will give the maximum allowed to Mr. Trump’s campaign and the Republican National Committee, but to spend the amount he contemplates would require donating through a “super PAC,” able to accept unlimited donations.

According to the Republicans familiar with Mr. Adelson’s planning, he and his advisers are still uncertain about which super PAC to use as their vehicle for the bulk of the contributions. They are wary of some of the current groups that purportedly exist to help Mr. Trump, who has been clear that he is uneasy with outside entities promoting his candidacy. At rallies, he has consistently criticized opponents who relied on super PACS, saying they were being bought by wealthy donors.

Mr. Adelson, 82, the chief executive of Las Vegas Sands, is among the world’s wealthiest individuals and has given hundreds of millions of dollars to Republican candidates and causes over the years. In 2012, he contributed at least $98 million to Republican efforts, according to a study by ProPublica. But that money went to 34 separate campaigns and groups.

Mr. Adelson is frustrated by congressional gridlock and believes the only way to affect the country’s political system is to ensure a Republican president is elected, say the Republicans familiar with his thinking.

While he may help some local or gubernatorial candidates, he is not planning to give much to congressional candidates or super PACs dedicated to keeping Republican control of the House and Senate, a substantial blow given the largess he has showered on them in recent elections.

Mr. Adelson and his wife, Miriam, met with Mr. Trump and his campaign manager, Corey Lewandowski, at the St. Regis Hotel in Midtown Manhattan while Mr. Adelson was in town for a gala dinner to benefit the World Values Network, an organization dedicated to disseminating Jewish values in politics, culture and media. The two moguls, who do not know each other well, had met earlier in the campaign.

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But their conversation last week marked the first time they had talked in person since Mr. Trump effectively won the nomination this month.

Mr. Trump assured the Adelsons that he was dedicated to protecting Israel’s security, an issue about which the couple are passionate.

The Adelsons contributed to Senator Ted Cruz of Texas during the Republican primaries, but kept a far lower profile than they had in 2012, when Mr. Adelson was a major benefactor of Newt Gingrich. Mr. Trump targeted Mr. Adelson on Twitter in October, writing that Mr. Adelson wanted to make Senator Marco Rubio of Florida “his perfect little puppet.”

But Mr. Adelson was plainly not bothered — or he at least forgave Mr. Trump. He told reporters last week he would get behind Mr. Trump and wrote an op-ed article in Friday’s Washington Post extolling his fellow casino owner.

“He is a candidate with actual CEO experience, shaped and molded by the commitment and risk of his own money rather than the public’s,” Mr. Adelson wrote, adding that Mr. Trump “has created a movement in this country that cannot be denied.”

A version of this article appears in print on May 14, 2016, on Page A15 of the New York edition with the headline: Casino Magnate Is Poised to Bolster Trump’s Bid. Order Reprints|Today's Paper|Subscribe